A mechanistic and spectroscopic study of two dioxo molybdenum enzymes is proposed, focusing on the various forms of enzyme encountered in the course of turnover and the characterization of their electronic structure. The subject enzymes, dimethylsulfoxide reductase (from Rhodobacter sphaeroides) and sulfite oxidase (from chicken liver), are members of a unique class of enzyme catalyzing oxygen atom transfer reactions that possess a MoO2 unit in their active sites. These enzymes catalyze a variety of essential metabolic reactions in all living organisms, including man. The manner in which oxygen atom transfer is accomplished in these systems, however, is poorly understood, particularly in comparison to the depth of current understanding of the mechanism of action of heme-, copper-, flavin- or pterin-containing enzymes that catalyze similar reactions. The principal goal of the proposed research is to redress the lack of mechanistic information regarding this important class of enzymes. It is known that, in contrast to the other systems mentioned above, the MoO2-possessing enzymes utilize water rather than dioxygen as the source of the oxygen atom incorporated into substrate when the reaction catalyzed is that of an oxidation; this reaction generates rather than consumes physiologically useful reducing equivalents. When the reaction catalyzed is that of a reduction, reducing equivalents are consumed, and the oxygen atom removed from substrate forms water. The dioxo molybdenum enzymes thus represent a unique solution to the type of chemistry being catalyzed. The primary aims of the proposed work are to compare and contrast the mechanistic and spectroscopic features of a MoO2 oxidase and a MoO2 reductase in order to gain further insight into the most fundamental aspects of catalysis by this class of enzymes.